1. Field of the Invention
The solder preform of the present invention is of particular utility in hermetically sealing a container for a semiconductor device such as an integrated circuit which must be protected from exposure to ambient atmosphere.
2. Description of the Prior Art
In the manufacture of semiconductor devices, there has been a requirement for hermetically sealing the container in which the active semiconductor device is housed, usually a cavity in a metallic or ceramic body. For this purpose, a metallic cover and a solder preform ring have been used. Initially, the cover, the solder preform, and the container were individually assembled and heated to fuse the solder to the cover and to the container as illustrated and described in U.S. Pat. No. 3,340,602 to Hontz. More recently, the solder preform ring has been preattached to the sealing cover and the resulting unit disposed over the container of the semiconductor device and heated to fuse the solder preform to the cover and to the container as illustrated and described in Applicant's U.S. Pat. Nos. 3,823,468 and 3,874,549. This latter method has the advantage that it permits the use of automatic machines for assembling the combination cover-solder preform units and the semiconductor container for fusing the solder unit as described.
In each of the foregoing methods of fabricating hermetically sealed semiconductor packages, it has been customary to use a solder preform formed of an eutectic alloy comprising substantially 80% gold and 20% tin, which has the advantages that it has a desirably high melting point of about 280.degree. C. and it has a high tensile strength, ensuring against breaking of the seal due to shocks or rough handling of the semiconductor package. While such solder preforms are eminently satisfactory in use, the recent extreme increase in the price of gold has made them very costly. In an effort to reduce the cost of such gold-tin solder preforms, solder alloys made from tin, silver, antimony, and other elements have been used to a limited extent but it has been found that they have severe functional limitations in that the melting point of the solder is undesirably low and the tensile strength of the solder joint is low compared to one made of a gold-tin eutectic alloy.
It is an object of the invention, therefore, to provide a new and improved solder preform .[.for use in hermetically sealing a container for a semiconductor device.]. which has the desirable characteristics of the gold-tin eutectic solder preform described above but which is must lower in cost.